Various different forms of equipment and machinery can be employed for mining and excavation operations, and typically it is the type of mining or excavation taking place, and the type of earth being mined or excavated, that dictates the type of equipment and machinery that is appropriate. The present invention is principally concerned with underground coal mining and with safety and maintenance issues relating to that form of mining. In relation to safety issues, one of the major safety difficulties in underground coal mining relates to fires or explosions that occur within the mine. These can occur due to the generation during mining of methane gas and coal dust (commonly known as mine dust), which can be trapped within the mine and is readily ignitable. Disadvantageously, the equipment used in coal mining can generate incendiary sparks and thus can cause fires or explosion. Therefore, it is important that all appropriate steps be taken to minimize or eliminate the production of sparks.
Equipment used to mine or excavate in hard earth, such as coal faces, can include rotary cutters, in which a rotating drum that carries a plurality of projecting cutting bits or picks, is brought into engagement with an earth face. The picks bite into the earth face as they rotate with the drum, to impact against and to dislodge or fragment earth from the face. This highly aggressive engagement between the picks and the earth face can result in spark production between them.
Cutting picks employed for coal mining generally have a hard cemented tungsten carbide tip that is fixed, usually by brazing, to a steel shank. Picks of this kind are disclosed in various prior art, such as U.S. Pat. No. 6,113,195, U.S. Pat. No. 4,725,098 and DE4226976. The tip of the picks can be either of the insert or cap style.
The insert style is shown in DE4226976, in which a greater section of the axial length of the tip is anchored within a bore of the shank, than extends out of the bore. The cap style is shown in U.S. Pat. No. 4,725,098, in which the tip has a broader base than the insert style tip and the base is located and brazed into a relatively shallow recess in the forward end of the shank. The present invention is applicable to cap style picks.
In picks of the above kind, sparks can be produced between the tungsten carbide tip and the earth face and also between the steel shank and the earth face, although there typically is greater likelihood of spark production between the steel shank and the earth face.
The highly aggressive environment in which the picks operate also results in wear of parts of the pick that come into contact with the earth being mined. Typically the shank of a pick is formed of a material that is softer than the tip of the pick and therefore the shank is more likely to be subject to wear than the tip and this is particularly the case closer to the tip.
An annular sleeve, attached to the shank of the cutting pick adjacent the cutting tip has been proposed for spark reduction, see for example in Applicant's U.S. Patent Publication No. 2005/0212345. The present invention relates to a similar form of cutting pick, but provides an arrangement to reduce the incidence of engagement of the sleeve with the earth face and/or with earth fragments dislodged during mining, in order to prolong the life of the sleeve against failure.